There is increasing interest in the development of strong lightweight ceramic products such as bricks and tiles.
Lightweight ceramic compositions which are strong and durable are in great demand for structural and semi-structural applications. Lightweight ceramics are desirable because of their dimensional stability, insulating properties, and inertness to corrosive and abrasive environments.
Ceramics are made in lightweight form by a variety of processes which commonly comprise one of the following steps as the means of inducing porosity:
1. the burnout method which involves the incorporation of combustible or volatilizable particles in a mixture of ceramic raw materials prior to firing;
2. firebloating a ceramic material to release a gas such as steam or an oxide of carbon or sulfur.
3. bonding lightweight inorganic particles with a relatively dense cementitious matrix, such as clay- or silica-bonded bubble alumina; or
4. dispersing or dissolving ceramic raw materials in a liquid, foaming the liquid, and then drying and firing the foam. In this method a preformed froth may be mixed with the dispersion or solution, or a compressed or condensed gas may be incorporated into the dispersion or solution and allowed to expand; or finally a gas may be generated in the liquid medium by a chemical reaction.
An example of a burnout process in the manufacture of insulating firebrick from a mixture of clay, sawdust, and gypsum. Firebloating processes are used to manufacture lightweight clay masonry or insulation units, foamed glass, expanded perlite, and expanded shale aggregate.
Known lightweight clay articles prepared by foaming water dispersions of clays are described by H. D. Foster, "Manufacture of Lightweight Products", Bull. Am. Ceram. Soc., 19 (12), 468-73 (1940) and by C. M. Nicholson and G. A. Bole, "Cellulated Ceramics for the Structural Clay Products Industry" Jour. Am. Ceram. Soc., 36 (4), 127-36 (1953). Known dispersion-foaming processes for clays employ organic polymeric or surface active foam stabilizers, such as cellulose ethers or esters, glues, gums, hydrolyzed vegetable and animal proteins, amines containing large hydrocarbon radicals, soluble salts of sulfonated hydrocarbons, and the like. Such foams have the disadvantage of loss of induced porosity and progressive shrinkage during drying and/or firing, resulting in inability to control density under commercial manufacturing conditions and often inability to achieve practical densities lower than 40 to 50 PCF (pounds per cubic foot).
Illustrative of prior art methods of manufacturing lightweight ceramic products, U.S. Pat. No. 1,761,108 describes a method for producing cellular building materials which involves the steps of admixing refractory clay with hydraulic cement and a gas generating powder such as aluminum powder; adding water to form a slip; casting the slip in molds; demolding and drying the formed blocks; and firing the blocks to vitrify the ingredients.
U.S. Pat. No. 3,310,614 described a method for producing extruded lightweight burned clay products having substantially uniformly distributed and regularly oriented flat flake-like voids. The method involves (1) mixing together 6-8 parts of clay or shale with 2-4 parts of seed hulls, (2) combining the admixture with 15-25 weight percent of water to form a stiff mud, (3) extruding the stiff mud into a columnar shape having the approximate cross-section of the desired clay product, (4) cutting the column shape into individual clay units, and (5) drying and firing the clay units to burn out the seed hull flakes and form a fired clay product having flat flake-like voids.
U.S. Pat. No. 3,700,470 describes a method for producing lightweight foamed solid shapes which involves (1) preparing an admixture of ceramic filler, sodium silicate, amphoteric metal powder and water, whereby the metal powder reacts with part of the sodium silicate to liberate hydrogen that expands the plastic mixture, (2) shaping the mixture, and (3) curing the shaped mixture by heating it at a temperature of at least 80.degree. C.
A typical foamed ceramic composition has a relatively low strength after firing, and the volume firing shrinkage makes it difficult to control and reproduce density under practical commercial manufacturing conditions. Also, any commercial process for making lightweight ceramics by a burnout or firebloating method can be a hazardous source of carbonaceous or sulfur-containing pollutants.
U.S. Pat. No. 3,944,425 discloses a method for providing lightweight foamed clay shapes which overcomes many of the disadvantages of the prior art technology described above. The method involves mixing, foaming and firing a composition comprising 20-95 parts of clay, 4-35 parts of hydraulic cement, 0.2-30 parts of inert particulate lamellar foam stabilizer, 0-76 parts of inert filler, and 21-70 parts of water, wherein all of the foaming action occurs between cessation of the mixing and about 10 minutes thereafter. The said U.S. Pat. No. 3,944,425 method is particularly suitable for slip casting of shaped clay bodies, as distinct from a stiff mud extruding procedure.
As indicated by the prior art references described above, investigative effort to develop improved lightweight ceramic compositions is a continuing objective.
Accordingly, it is a main object of this invention to provide a lightweight ceramic composition which has exceptional strength and durability at a density below about 105 pounds per cubic foot.
It is a further object of this invention to provide a deformable cellular clay body which can be shaped and sized by extrusion into a self-supporting columnar matrix.
It is a further object of this invention to provide a method of manufacturing foamed structural clay products, for which method conventional extrusion brick-making equipment may be readily adapted.
It is a further object of this invention to provide a deformable cellular clay body which can be fired to a lightweight ceramic matrix without the evolution of noxious volatiles.
It is a further object of this invention to provide foamed structural clay products by a method which employs less than about 50 percent of the drying and firing energy requirements of conventional brick manufacturing methods.
It is another object of this invention to provide a structural cellular clay product which has pores which are substantially uniform and non-interconnected.
It is still another object of this invention to provide lightweight foamed structural clay products with at least one relatively dense, impervious and abrasion-resistant surface.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.